A variety of copolymers of allylic and ethylenic monomers are well known, and many are particularly useful in the coatings industry. Examples are copolymers of allylic and vinyl aromatic monomers (such as styrene-allyl alcohol (SAA) copolymer), and copolymers of allylic and acrylate monomers. These copolymers offer performance advantages in end uses such as wood coatings, automotive topcoats, and powder coatings.
SAA copolymers (see U.S. Pat. Nos. 2,630,430, 2,894,938, and 2,940,946) are resinous polyols useful for polyesters, fatty ester emulsions, alkyd and uralkyd coatings, melamines, and polyurethanes. They can be made in a batch process by charging a reactor with styrene, allyl alcohol, and a free-radical initiator, and heating the mixture at a temperature effective to polymerize the monomers. Recently, we described a semi-batch process for making these copolymers (see U.S. Pat. No. 5,444,141). We showed that yields improve significantly when the free-radical initiator is gradually added to the reaction mixture. More recently, we showed that good yields of SAA copolymers having higher styrene contents (and lower hydroxyl number) could be made using a similar process (see copending application Ser. No. 08/888,489, filed Jul. 8, 1997).
Like SAA copolymers, hydroxy-functional acrylic copolymers react with a wide assortment of crosslinking agents to give coatings. Hydroxyl functionality is usually incorporated by using a hydroxyalkyl acrylate monomer or, as we showed more recently (see U.S. Pat. Nos. 5,475,073 and 5,525,693), by using a hydroxy-functional allyl monomer such as allyl alcohol or an alkoxylated allyl alcohol.
The allylic copolymers described above are often best made using a "semi-batch" process because of the reactivity difference between the allyl monomer (sluggish), and the ethylenic monomer (fast). More specifically, all of the allylic monomer is usually present in the reactor at the start of the polymerization, while most of the ethylenic monomer and free-radical initiator are added to the reactor gradually during the course of the polymerization. We found that this "gradual addition" technique gives higher yields compared with the typical batch process. For example, in U.S. Pat. No. 5,444,141, we showed that gradual addition boosted yields of SAA copolymers by 30-50%.
Another common feature of these polymerizations is that the reaction temperature is normally kept constant during addition of the ethylenic monomer, which is preferably added at a decreasing rate (see, e.g., Example 1 of U.S. Pat. No. 5,444,141, and Example 1 of U.S. Pat. No. 5,475,073). In each case, the reaction temperature is kept constant throughout the addition of the ethylenic monomer.
In spite of the progress made earlier in obtaining higher yields of allylic/ethylenic copolymers, particularly SAA copolymers, there is still room for improvement since even the best yields are only 50-70%. For example, the process of U.S. Pat. No. 5,444,141 (Example 1) gives a 40% yield of SAA copolymer, and the process of U.S. Pat. No. 5,475,073 (Example 1) gives a 67% yield of hydroxy-functional acrylate copolymer. A preferred process would give even higher yields of copolymers having desirable molecular weights, hydroxyl numbers, and allyl monomer contents. Ideally, the process would be cost-effective and easy to perform with conventional equipment.